Metal substrate double sided circuit board

ABSTRACT

Disclosed is a circuit board and a process for the manufacture thereof providing a circuit board comprising a metal core having parallel first and second major faces and exhibiting high thermal and electrical conductivity. The circuit board includes electrical insulating layers of thermally conductive, dielectric material applied to the first and second major faces of the metal core. Protecting the dielectric layer and copper conductors is a solder mask layer applied to the dielectric layers and forming outward facing major surfaces. A plurality of insulated and grounded vias having electrically conductive interior rings connecting the major surfaces are provided through the board. Conductive sleeves within the vias are either electrically insulated from the metal core by dielectric material or in electrical contact to the metal core for grounding.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to electronic circuit boards and more particularlyto multi-layer circuit boards exhibiting high thermal conductivity forheat dissipation and providing for double sided mounting of circuitelements to achieve high circuit element volumetric density.

2. Description of the Related Art

Contemporary electronic circuits are conventionally fabricated bymounting circuit elements and integrated circuit packages on circuitcards or boards. The boards electrically interconnect the variouspassive and active circuit elements and integrated circuit packages intoone or more functional units. Conventional multi-layer boards aremanufactured by laminating together layers of, for example, glassreinforced epoxy, and have copper conductors affixed to one or moremajor surfaces of the board. Circuit elements may then be attached tothe conductors to complete the device.

While laminate boards composed of non-thermally conductive materials aresuitable for many low density applications, such boards are less thanoptimum for application to power supplies and other applications havinga high density of circuit elements. Power supplies are often rated onthe basis of watts per unit volume. Components such as transformers andfilters can be reduced in size if operating frequency is increased.Higher operating frequencies though place a premium on short, lowinductance connections between circuit elements. It is also preferableif use of both sides of a circuit board for support of devices isallowed. In this way the smaller devices are brought physically closertogether. However, reduced size reduces the surface area of the devicefrom which to radiate heat. Close physical proximity to other heatgenerating devices complicates the heat dissipation process. As aconsequence, heat removal, which is already a problem for boards usedfor power supplies, promises to become still more difficult as operatingfrequencies are increased from the 50 to 200 kilohertz range to rangesin excess of 1 Megahertz.

The use of insulated metal core circuit boards to increase heatdissipation is known. Although high thermal conductivity to the metalcore is also known, it has been limited to use on single sided boards.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved circuit board.

It is another object of the invention to provide high circuit elementvolumetric density.

It is still another object of the invention to provide a circuit boardexhibiting high thermal conductivity for heat dissipation.

It is another object of the invention to provide said circuit boardallowing two sided mounting of devices.

The invention provides a circuit board comprising a metal core havingparallel first and second major faces and exhibiting high thermal andelectrical conductivity through the core. The circuit board includeselectrical insulating layers of thermally conductive, dielectricmaterial applied to first and second major faces of the metal core.Protecting the surfaces of the circuit board is a solder mask layerapplied on top of the Cu circuitization and dielectric layers that formoutward facing major surfaces. A plurality of insulated and groundedvias having electrically conductive interior rings connecting thecircuit layers are provided through the board. Conductive sleeves withinthe vias are either electrically insulated from the metal core bydielectric material or in electrical contact to the metal core forgrounding.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself however, as well as apreferred mode of use, further objects and advantages thereof, will bestbe understood by reference to the following detailed description of anillustrative embodiment when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a perspective section view of a metal core circuit board withvias supporting devices on both major surfaces; and

FIGS. 2-15 are a series of sectional views illustrating stages offabrication of the high thermal conductivity circuit board of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

With reference now to the figures and in particular with reference toFIG. 1, there is depicted a double sided circuit board 10, possessingparallel major faces 11 and 13. Circuit board 10 is constructed on athermally and electrically conductive core 12. Core 12 is preferablycopper, although other metals or metal laminates may also be used. Core12 provides heat transfer and heat dissipation for devices mounted toboard 10 and also serves as a circuit ground plane.

Dielectric layers 14, are applied to the parallel major surfaces of core12. Dielectric layers 14 are made of thermally conductive material suchas aluminum nitride (AIN), boron nitride (BN), diamond, diamond-likecarbon, or ceramic polymer composite materials. The composite materialsshould be loaded with between 50% and 80% by volume of the thermallyconductive ceramic material. The thickness of dielectric layers 14depends upon the application of board 10 and the dielectric constant andvoltage breakdown of the material used to construct the dielectriclayers. Resting on the dielectric layers 14, below major faces 11 and13, are a plurality of electrical leads 20, that are covered byprotective solder mask layers 16. The solder mask layers provide forenvironmental protection of the board 10, the dielectric layers 14, andthe electrical leads 20. Exposed areas of the electrical leads 20 notbeing covered by solder mask 16 form portions of major faces 11 and 13.Exposed portions of leads 20 provide mounting areas 23 for devices suchas surface mounted integrated circuit packages 22, pin thru holecomponents, connectors, and other devices including transformers 24.

Dielectric layers 14, and electrical leads 20, are fabricated usingcombinations of lamination and plating methods, or combinations ofvarious vapor deposition, plasma spray or sputtering technologies, andplating methods. When using a combination of lamination and platingmethods, electrical leads 20, are built up in two layers, 26 and 28,with layer 26 formed on dielectric layer 14 being preferably composed ofcopper foil, and layer 28 being made of copper plating. When using vapordeposition, plasma spray or sputtering technologies, and platingprocesses for dielectric layers 14 and electrical leads 20, layer 26 ondielectric layer 14 is preferably a copper seed layer processed viasputter or vapor deposition technology, while layer 28 is processed viaplating with copper as the preferred metal. Additional layers 30,comprised preferably of Au over Ni can also be added via plating toselected mounting areas 23, if required for component attach purposessuch as wirebonding.

A plurality of vias are provided through circuit board 10, includinginsulated vias 32, and grounded vias 34. Insulated vias 32, include abore 36, that provides for connection between electrical leads 20 onmajor faces 11 and 13. An electrically insulating liner 44 isolates viabore 36 from board core 12 and can be made from the materials that makeup dielectric layers 14, or from epoxies, polyimides and other resins.Applied to liner 44 is a copper sleeve 42, that provides electricalconnection to leads 20. A solder coating 38, may also be applied overthe copper sleeve 42 if desired.

Grounded via 34, includes a bore 36, that allows for connection ofelectrical leads 20 to metal core 12. To provide an electrical pathwayto ground from electrical leads 20 to metal core 12, a copper sleeve 42is applied directly to the via wall of bore 36. A solder coating mayalso be applied over copper sleeve 42, if so desired.

Turning next to FIG. 2, the first stage in the process for fabricatingcircuit board 10 is shown, beginning with a copper plate 50 having athickness suitable for power dissipation. Typical ranges include 0.02 to0 .08 inches which will form metal core 12 in the final product.

Referring to FIG. 3, clearance holes 52 for surface to surfaceconnection vias (insulated vias 32) have been drilled in plate 50.

Next, in FIG. 4, clearance holes 52 have been filled with plugs 54 ofhole fill compound. Plugs 54 may be non-thermally conductive orthermally conductive composite materials. This material can be the samematerial as in surface layer 14 or different.

In FIG. 5, plate 50 and plugs 54 have been laminated with a thermallyconductive B-stage prepreg film to form dielectric layers 14 therebycovering the major faces of plate 50 and enclosing plate 50 as metalcore 12. A layer of copper foil 56 has been laminated on top of eachdielectric layer 14. Both the dielectric and copper are laminatedsimultaneously. The hole fill process FIG. 4 and the lamination of thesurface dielectric 14 in FIG. 5 and the copper foil can also be carriedout simultaneously. Dielectric layers 14 may be formed from compositematerials such as an epoxy or polyimide filled with 50 to 80% by volumewith a thermally conductive dielectric such as boron nitride, diamond,aluminum nitride, or a combination of the foregoing materials.Alternative process steps for deposition of coatings of these materialsexist.

Aluminum nitride or boron nitride may be low temperature arc vapordeposited. For application of aluminum nitride or boron nitride to ametal surface, a high ion energy (60-100 eV) and high degree of plasmaionization (approximately 90%) are used. This leads to formation ofdense, extremely adherent coatings on a metal substrate. In general, theformation of nitride coatings is done through introduction of nitrogengas into the plasma which interacts with a base material, such asaluminum. Following deposition of the nitride coating, circuitization(i.e. the formation of electrical leads) may be accomplished bysputtering of copper or electroless copper as a seed and subsequentbuild up of copper using electrolytic plating techniques.

Diamond and diamond-like coating of carbon may be formed by chemicalvapor deposition processes which allows for uniform application on thesurfaces of the metal plate 50 and in the clearance holes 52. 4Circuitization is then achieved on the card through sputter depositionof copper seed and electrolytic copper plating.

Next, in FIG. 6, clearance holes 58 and 60 have been drilled through thebody of the board. Clearance hole 58 is drilled centered on plug 54.However, hole 58 is narrower than plug 54 leaving an insulating collar44 lining hole 58. Clearance hole 60 is drilled through metal core 12and is accordingly in contact with the core along its surface. Plasmadesmearing may be applied to clean clearance holes 58 and 60 at thispoint.

FIG. 7 illustrates application of a layer 61 of photoresist (preferably0.0010 and 0.0020 inches in depth) and a glass or mylar photomask 62outlining circuitization paths for the final circuit board 10.

FIG. 8 illustrates the state of the board 10 after the photoresist layer60 has been exposed to ultraviolet light through the photomask 62, andthe unexposed photoresist has been removed with an appropriate solvent.

FIG. 9 illustrates a stage of fabrication of board 10 after chemicaldeposition of seed within clearance holes 58 and 60 followed byelectrolytic deposition of copper 64 on the exposed copper foil 56 andin the clearance holes 58 and 60. The layer 28 of electrical leads 20and copper sleeve 42 are now in place.

In FIG. 1 0, solder plate 66 has been applied to all exposed coppersurfaces including within clearance holes 58 and 60.

In FIG. 11, all exposed photoresist 60 has been stripped.

In FIG. 12, all copper foil 56 formerly underlying the exposedphotoresist 60 has been removed by chemical etching, leaving bottomlayers 26 for electrical leads 20 now in place under layers 28.

Next, FIG. 13 illustrates board 10 after the solder plate has beenstripped. An alternative method for fabricating the electrical leads 20would be to electrolytically plate copper foil surface 56, and the holes58 and 60. Then form the desired electrical leads 20 and the platedholes 32 and 34 by a subtractive process (not shown) well known in theart of fabricating circuit boards.

In FIG. 14, a solder mask 70 has been applied to the major faces of theboard, exposed to ultraviolet radiation through a photomask (not shown),and developed to provide protective layers 16 and exterior majorsurfaces 11 and 13. The unexposed solder mask is developed (removed)from copper pads which will then be solder plated or nickel/gold plated.

FIG. 15 illustrates a nearly completed board 10 ready to receivedevices. Solder plate (0.0010 to 0.0030 inches in thickness) has beendeposited to form tubes 38 within vias 32 and 34 and to provide surfacemount and pin in hole mounts for devices to be connected to the board.Edge connector pads 30 or direct chip attach pads (not shown) have beencompleted by the addition of a nickel/gold plating (depth approximately0.0002 inches).

Circuit boards in accordance with the present invention provide highsubstrate thermal conductivity, eliminating the need to use separateheat spreaders for many applications. They also allow forvia-through-hole, double sided, high density packaging therebysignificantly reducing physical card dimensions for power supplyapplications. Also improved are pin-in-hole and multi-layer packagingcapability. The boards are characterized by ability to withstand highvoltages before breakdown through the use of high breakdown voltagedielectric material. Both surface conductors and the metal core exhibitlow sheet resistance for high current carrying capability. The boardallows direct attachment of integrated circuits as well as supportinguse of surface mount and pin-in-hole technology. Boards constructed inaccordance with the teachings herein exhibit excellent mechanicalstrength. Applications for the boards include power regulators,converters, motor drivers, print head drivers and heat sinks forcomputers and other electronic packaging applications.

While the invention has been particularly shown and described withreference to a preferred embodiment, it will be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. A circuit board comprises:a metal core havingparallel first and second major faces and exhibiting high thermal andelectrical conductivity; dielectric layers of thermally conductivematerial applied to the first and second major faces of the metal core;a plurality of insulated and grounded vias through metal core and thedielectric layers; an electrically conductive sleeve within eachinsulated and grounded vias, wherein the electrically conductive sleevesin the insulated vias are electrically insulated from the metal core bydielectric material and the electrically conductive sleeves in thegrounded vias are in electrical contact to the metal core for grounding;and electrically conductive leads connected to selected ones of theelectrically conductive sleeves and applied to each of the dielectriclayers and spaced thereby from the metal core,
 2. The circuit board ofclaim 1, wherein the dielectric layers are aluminum nitride, boronnitride, diamond or ceramic polymer composites.
 3. The circuit board ofclaim 2, wherein the metal core is copper, aluminum or anodizedaluminum.
 4. The circuit board of claim 3, and further comprising:firstand second solder mask layers applied over the conductive leads anddielectric layers and forming outward facing major surfaces of thecircuit board; and electronic components mounted to each major surfaceof the circuit board.
 5. The circuit board of claim 1, wherein thedielectric layers are formed of an epoxy or polyimide matrix filled with50% to 80% by volume with a thermally conductive dielectric.
 6. Thecircuit board of claim 5, where the thermally conductive dielectric isboron nitride, aluminum nitride, diamond, diamond-like carbon or acombination of two or more of the forgoing materials.